Fever in a baby, and when it matters

Fever is the most common reason parents seek urgent medical help, and the single most important variable in deciding what to do is the baby's age.

The age rules

Stated first because they matter most.

Under three months with a temperature of thirty-eight degrees or above: seek medical assessment immediately, day or night.

Young infants can have serious bacterial infection with few other signs, their immune systems are immature, and guidelines in most countries mandate urgent assessment and usually investigation.

This is not a wait-and-see situation.

Three to six months with a temperature of thirty-nine or above: seek medical advice promptly.

Over six months: the number matters less than how the child is — their alertness, colour, breathing, feeding and responsiveness.

A cheerful child with a high temperature is generally less concerning than a listless child with a modest one.

Measuring it

Method matters.

For under-fours, an electronic thermometer in the armpit is recommended in several guidelines, or an infrared tympanic thermometer for older infants.

Forehead strip thermometers are not reliable.

Rectal and oral measurement is not recommended for infants in most current guidance.

Do not adjust the reading with formulas; report what the device says and which method was used.

The red flags

Signs requiring emergency assessment at any age.

A rash that does not fade when pressed with a glass — though sepsis and meningitis can occur without any rash, so its absence is not reassurance.

Difficulty breathing: grunting, rapid breathing, drawing in below the ribs, flaring nostrils, or pauses.

Blue, pale, mottled or ashen skin.

Being difficult to wake, unusually drowsy, or not responding normally.

A weak, high-pitched or continuous cry.

A bulging fontanelle.

Neck stiffness, or a fit or seizure.

Cold hands and feet with a hot body.

Not feeding, or significantly reduced wet nappies.

And a parent's sense that this illness is different from previous ones, which clinical guidance explicitly recognises as significant.

What fever is doing

Worth understanding because it reduces the urge to suppress it reflexively.

Fever is a regulated response to infection rather than a malfunction, and it is not itself harmful at the temperatures seen in common childhood illness.

The height of the temperature correlates poorly with the seriousness of the illness in children over three months.

Which is why current guidance advises treating discomfort rather than treating the number.

Managing it

Paracetamol or ibuprofen may be given for a child who is distressed, at the dose for their weight and age, following the product instructions.

Do not give aspirin to children.

Do not routinely alternate the two medicines, and do not give both simultaneously as a matter of course — guidance advises using one, and considering the other only if the child remains distressed.

Do not use them purely to reduce a temperature in a comfortable child.

Do not tepid sponge or cold bathe, which is no longer recommended.

Do not underdress to the point of shivering or overwrap.

Offer fluids frequently and in small amounts.

And check on the child during the night while they are unwell.

Febrile seizures

Frightening and usually benign.

They occur in a small proportion of young children with fever, most commonly between six months and three years.

During one: put the child on their side on a safe surface, do not restrain them, do not put anything in their mouth, note the time, and call emergency services for a first seizure or one lasting more than five minutes.

Most are brief, and most children who have one never have another.

They do not cause brain damage and the great majority of children have no long-term consequences.

What to have at home

A reliable thermometer.

Infant paracetamol and ibuprofen suspensions, in date.

The number for your out-of-hours service and the national health advice line.

And oral rehydration sachets, which are useful for vomiting and diarrhoea.

General information only, not medical advice. Seek immediate medical assessment for any fever in a baby under three months, and urgent care for any red flag symptom.